The present disclosure relates broadly to the field of solar power generation used to produce electricity. More particularly, this disclosure relates to a dual-exposure or two-sided heat absorption panel, and a solar receiver including one or more of such panels. These solar receiver designs can be used with Concentrated Solar Tower technology, also known as Concentrating Solar Power (CSP) technology to harness the sun's energy to produce “green” electricity.
A solar receiver is a primary component of a solar energy generation system whereby sunlight is used as a heat source for the eventual production of superheated high quality steam that is used to turn a turbine generator, and ultimately produce electricity using the Rankine cycle or provide steam for other thermal processes.
Generally, the solar receiver is positioned on top of an elevated support tower which rises above a ground level or grade. The solar receiver is strategically positioned within an array of reflective surfaces, namely a field of heliostats (or mirrors), that collect rays of sunlight and then reflect and concentrate those rays back to the heat absorbing surfaces of the solar receiver. This solar energy is then absorbed by the working heat transfer fluid (HTF) flowing through the solar receiver. The reflective surfaces may be oriented in different positions throughout the day to track the sun and maximize reflected sunlight to the heat absorbing surfaces of the receiver.
The solar receiver is an assembly of tubes with water, steam, molten salts, or other heat transfer fluid (HTF) flowing inside the tubes. The HTF inside the tubes of the receiver absorbs the concentrated solar energy, causing the HTF to increase in temperature and/or change phases, so that the HTF captures the solar energy. The heated HTF is then either directly routed to a turbine generator to generate electrical power or is indirectly routed to a storage tank for later use.
Solar receiver designs typically include an arrangement of panels with vertically oriented tubes, i.e. tube panels, along with a support structure for maintaining the tube panels in place and other associated equipment (pumps, pipes, storage vessels, heat shields, etc.). In conventional designs, the solar receiver has a square, rectangular, or circular cross-section (in a plan view from above). The tube panels are arranged on the exterior of the cross-section, so that the solar energy from the heliostats is directed at (and absorbed by) only one face of a tube panel. This is illustrated in, for example, U.S. patent application Ser. No. 12/605,241, which is entitled “Shop-Assembled Solar Receiver Heat Exchanger” and is assigned to Babcock & Wilcox Power Generation Group, Inc., and which is hereby fully incorporated by reference herein.
In this regard, FIG. 1 is a plan view (i.e. viewed from above) of one solar receiver design 100 discussed above, which has four tube panels 110, 120, 130, 140, arranged as a square. Each tube panel has one exterior face 112, 122, 132, 142 which is exposed to solar energy from heliostats, and one interior face 114, 124, 134, 144 which is not exposed to such solar energy.
The interior non-absorbing face of a tube panel usually has a buckstay system that supports the tube panels against high wind, seismic forces, and thermally induced forces. The buckstay system typically includes “I” beams or other structural steel shapes that are clipped onto the tube panel in such a way that the tube panel can expand independent of the support structure itself and independent of the other tubes and panels. Clips are usually welded to the tubes so that the tube panel can move relative to the stationary support structure when heat is applied to the tubes, yet the support structure can still provide rigidity to the tube panel. On a solar receiver, the tubes in the tube panel are not welded together along their axes (i.e. membrane construction) as in a fossil fuel fired boiler, but are of loose construction. This allows the tubes to expand independently of each other when heat is applied. As a result, each tube must have a clip to attach to the buckstay at a support elevation.
One problem that results due to only one face of a tube being exposed to solar energy is that a temperature differential arises between the exposed hot face and the non-exposed cold face. This results in differential expansion between the hot and cold faces of the tube, which causes the tube to bow. The severity of bowing depends on the magnitude of the temperature differential and the rigidity of the tube panel. Because the clip connecting the tube to the buckstay keeps the tube in place at the support elevation, bowing occurs between support elevations. This creates high compressive stress on the heated side of the tube at each support elevation.
Due to daily heating and cooling of the tubes during startup, shutdown, and cloud passages, such stresses are cyclic, which can eventually lead to fatigue failure. For receivers that use molten salt as the HTF, impurities in the molten salt can also cause corrosion, which can be exacerbated where stress is located.